As the most important primary energy source in our country, coal-fired power generation plays a critical role in industrial production and household consumption. Multiple pollutants are released in the coal-fired process. Typical pollutants, such as SO2, NO and NO2, lead to serious environmental problems such as acid rain, photochemical smog and ozone layer destruction via a series of chemical reactions in the atmosphere. These serious environmental problems are harmful for human health. At present, the conventional selective catalytic reduction technology and wet flue gas desulfurization technology is commonly used to remove nitrogen oxides and sulfur oxides in coal-fired flue gas of power plant, respectively. There are still some serious disadvantages in these technologies such as large occupation, high investment and operating costs, difficulties in handling spent catalyst as solid waste. Consequently, it is of great importance to further develop simultaneous removal technology in one reactor or one set of reactors. Our group proposed a novel process for SO2 and NO removal in one set of reactors, that is "simultaneous removal of SO2 and NO with H2O2/Fe-based material coupled with ammonium-based absorbent". The whole process is described as follows. At first, SO2 is removed with ammonium-based absorbent in the first absorption tower. The flue gas after desulfurization is blended with the vaporized H2O2 and pass through a catalytic reactor. The NO in flue gas is oxidized into the high-valent nitrogen oxides. In the secondary absorption tower, the high-valent nitrogen oxides are removed with ammonium-based absorbent from the first absorption tower. In the aspects of liquid absorption and gas phase oxidation, the theoretical and experimental researches on the novel technique are carried out in this paper, respectively.
(1) In this novel process, the SO2 and NO2 in the flue gas are easily removed by ammonium-based solution rather in scrubber but NO is rarely removed. In the self-designed bubbling scrubber, the effect of operation parameters on the SO2 and NO2 removal with ammonium-based solution is investigated and the mechanism is revealed with the thermodynamic calculation and kinetic model. For the desulfurization experiments in the first tower, experimental results indicate that both 10 wt.% urea solutions and 5 wt.% urea / 5 wt.% ammonium sulfite solutions have a good performance on SO2 removal rather than NO removal. For the NO2 removal experiments in the second tower, the content of ammonium sulfite in ammonium-based solution is the crucial factor in NO2 absorption. The characterization of product indicates that the reduction reaction between NO2 and ammonium sulfite is the main approach for NO2 absorption. On the basis of NO2 removal experiments, the mechanism is intensively discussed in aspects of thermodynamics and kinetics. In the aspect of thermodynamic study, the results of thermodynamic calculations in different temperature indicate that only NO and NO2 are stable in the range of 20℃ ~ 90℃ and the direct reduction of NO2 or HNO2 with ammonium sulfite is the main NO2 removal reaction. The NO2-H2SO3-NH3-H2O reaction system potential-pH plot (at 50℃) showed that, in the pH range from -7 to 14, both NO2 in gas phase and 〖"NO" 〗_"2" ^"-" in liquid phase can be removed by SO2, producing 〖"SO" 〗_"4" ^"2-" and N2. In the aspect of kinetic study, the gas-liquid mass transfer parameters of reactor are determined by the chemical method and Danckwerts plotting. On the basis of the two-film theory, the NO2 absorption rate equation is established. The NO2 absorption rate and reaction rate is calculated. The NO2 removal process is analyzed with Hatta number. According to the experimental results, the NO2 absorption with 5wt.% urea / 5wt.% ammonium sulfite is verified as a fast pseudo-first-order reaction. An empirical expression for the pseudo-first-order reaction rate constant for NO2 removal is established. The empirical expression for the first order reaction rate constant and NO2 absorption rate are obtained by fitting experimental data. The NO2 absorption rate equation can be used to simulate the NO2 absorption rate in 5wt.% urea / 5wt.% ammonium sulfite in bubbling bed.
(2) In this novel process, NO can be efficiently oxidized and removed in H2O2/Fe2O3 heterogeneous Fenton reaction coupled with ammonium-based solution and the catalytic mechanism is intensively discussed. The effects of operation parameters on NO removal are intensively studied in a self-designed bench-scale device. The denitrification efficiency keeps 78% in 2 hours, under the operation conditions where the flue gas flow rate is 1.5 L/min, NO concentration is 530 ppm, O2 concentration is 7%, catalyst dosage is 2 g, H2O2 concentration is 2 mol/L, H2O2 feeding rate is 5mL/h, flue gas preheating temperature is 140℃, vaporization temperature is 140℃, catalytic temperature is 140℃. On the basis of material balance calculation, the possible denitrification process is speculated. The NO removal experiments with Fe2O3 / typical supports (Al2O3 and SiO2), typical Fe-based spinel (ZnFe2O4, NiFe2O4 and CuFe2O4) and typical Fe-based perovskite (LaFeO3 and La0.85FeO3) is premiere presented. It is indicated that the Fe-based perovskite has a better performance on NO removal than other catalysts. The NO oxidation with H2O2 / Fe2O3 is studied in aspects of free radical capture, thermodynamic calculation, analysis of gas-liquid two-phase products, the physicochemical property of catalyst and kinetics study. On the basis of experimental results, the catalyst mechanism on H2O2 / Fe2O3 system is studied and investigated in depth. The free radical capture and scavenger experiments indicate that the ?OH plays a critical role in NO removal. The thermodynamic calculation and the analysis of product show that the main oxidation products are HNO3 and NO2. The characterization (XRD, FTIR, SEM-EDX and XPS) of fresh and spent catalyst indicates the changes of physicochemical property. The XRD characterization indicates that the crystal structure of catalyst is robust in 2 hours’ reaction. The XPS spectra of catalysts show that the catalytic decomposition of H2O2 over Fe2O3 follows the Haber-Weiss mechanism. According to 4 kinds of catalytic kinetic model (adsorption - reaction - desorption) and the fitting of experimental data, it is proved that the NO oxidation follows the Eley-Rideal catalytic reaction model, that is, the H2O2 adsorbed on the catalyst surface and subsequently, NO is oxidized by active product.
(3) Due to the good performance of typical Fe-based perovskite in NO removal, La1-xCaxFeO3 (x=0, 0.1, 0.3 and 0.5) is prepared and studied in NO removal experiments. The structure-activity relationship is established and the two kinds of surface catalytic mechanism is elucidated. The NO removal experiments and oxidaiotn product analysis show that, with the Ca doping amount increased, NO concentration in the oxidized flue gas increases while the NO2 and HNO3 concentrations decrease, leading to the decreased NO removal efficiency. It is clear that the doped Ca can weaken the NO removal ability of La1-xCaxFeO3 (x=0, 0.1, 0.3 and 0.5). According to the ESR spectra and the KMnO4 titration test, the doped Ca accelerates H2O2 decomposition without produce ?OH. According to the characterization of catalyst surface physicochemical property, the surface oxygen vacancy of La1-xCaxFeO3 (x = 0.1, 0.3, and 0.5) increased a lot with the Ca doping amount increased. According to the intensive discussion and correlation of catalyst physicochemical property, ?OH generation concentration, H2O2 decomposition rate and NO removal efficiency, the inhibition mechanism of doped Ca on NO removal can be elucidated as follows. On the La1-xCaxFeO3 (x=0, 0.1, 0.3 and 0.5), the decomposition of H2O2 over surface oxygen vacancies via superficial mechanism produces O2 and H2O instead of ?OH production via Habber-Weiss mechanism, which is the main factor for the declined NO removal efficiency.
(4) For the high-sulfur flue gas, the desulfurization and denitrification from the with H2O2 / Fe2(SO4)3 coupled with ammonium-based solution is carried out and the improvement mechanism of high SO2 concentration on NO removal is studied. Under the conditions of 2000 ppm SO2, 500 ppm NO, 7% O2, catalyst temperature of 140℃, H2O2 concentration of 1 mol/L, H2O2 feed rate of 5 mL/h and 2 g catalyst, the SO2 and NO efficiency is 99.8% and 92.5%, respectively. The comparison between separate NO removal and simultaneous SO2 and NO removal indicates that the additive SO2 is benefit for the NO removal. The XPS spectra indicate that SO2 promotes formation of oxygen vacancy on catalyst, leading to the reduction of Fe(III) into Fe(II). Compared with the ?OH generation via Haber-Weiss mechanism, the catalytic decomposition of H2O2 over "≡Fe" ("II" )"-OH" performs a higher ?OH generation rate and consumed less H2O2, indicating the promotion of SO2 on NO removal. With respect to the decreased NO removal efficiency in a 12 hours experiments, the results of SEM-EDX indicated that the agglomeration of Fe2(SO4)3 particle may be the main reason.